New performance levels for arrays of nanowires from III-V materials directly grown on graphene have been reached by researchers at the University of Illinois at Urbana-Champaign.
Nanowire semiconductors hold great promise for applications involving light, like solar cells or lasers.
“Over the past two decades, research in the field of semiconductor nanowires has helped to reshape our understanding of atomic-scale crystal assembly and uncover novel physical phenomena at the nanometer scale,” said engineering professor Xiuling Li.
New Solar Cell Architecture
A recent paper from the researchers is the first report of a new solar cell architecture based on dense arrays of coaxial p-n junction InGaAs nanowires on InAs stems grown directly on graphene without any metal catalysts or lithographic patterning.
“In this work, we have overcome the surprising structure (phase segregation) and successfully grown single phase InGaAs and demonstrated very promising solar cell performance,” says first author Parsian Mohseni.
“Depending on the materials, nanowires can be used for functional electronics and optoelectronics applications,” Mohseni explained. “The main benefits of this III-V photovoltaic solar cell design are that it is fairly low-cost, substrate-free, and has a built-in back side contact, while being conducive to integration within other flexible device platforms.”
The nanowire array is readily lifted from its graphene base and can be transferred to other bases for bendable device applications
van der Waals Epitaxy
The research group used a technique known as van der Waals epitaxy to grow nanowires from the bottom up on a sheet of graphene.
Into a chamber where the graphene sheet sits are then pumped gases, containing indium, gallium, and arsenic, triggering the nanowires self-assembly, growing by themselves into a dense carpet of vertical wires across the graphene’s surface
In earlier work published in Nano Letters in 2013, the researchers discovered, using a graphene sheet, that InGaAs wires grown on graphene spontaneously segregate into an indium arsenide (InAs) core with an InGaAs shell around the outside of the wire.
To boost the materials’ efficiencies for solar power conversion, the researchers bypassed the unique van der Waals epitaxy induced spontaneous phase segregation by inserting InAs segments in between.
The resulting ternary InGaAs NW arrays are vertical, non-tapered, controllable in size, height, and doping, and broadly tunable in composition thus energy for monolithic heterogeneous integration with 2D van der Waals sheets including graphene.
Record Conversion Efficiency
Under air mass 1.5 global solar illumination, the core-shell In0.25Ga0.75As (Eg ~ 1.1 eV) nanowire arrays on graphene showed conversion efficiency of 2.51 %, which is a new record for substrate-free, III-V NW-based solar cells.
“Although InGaAs is far from being the optimum bandgap materials for high efficiency solar cells, the direct epitaxy on graphene platform established here has significant implications for a wide variety of III-V compound semiconductor NW based solar cells on graphene, as well as light emitters and multi-junction tandem solar cells, all of which can be released for flexible applications,” Li said.